Feedbacks of Vegetation on Summertime Climate Variability over the North American Grasslands: 1. Statistical Analysis

Abstract

Feedbacks of vegetation on summertime climate variability over the North American Grasslands are analyzed using the statistical technique of Granger causality. Results indicate that NDVI (normalized difference vegetation index) anomalies early in the growing season have a statistically measurable effect on precipitation and surface temperature later in summer. In particular, higher means and/or decreasing trends of NDVI anomalies tend to be followed by lower rainfall but higher temperatures during July through September. These results suggest that initially enhanced vegetation may deplete soil moisture faster than normal, and thereby induce drier and warmer climate anomalies via the strong soil moisture/precipitation coupling in these regions. Consistent with this soil moisture/precipitation feedback mechanism, interactions between temperature and precipitation anomalies in this region indicate that moister and cooler conditions are also related to increases in precipitation during the preceding months. Because vegetation responds to soil moisture variations, interactions between vegetation and precipitation generate oscillations in NDVI anomalies at growing-season time scales